1. Field of the Invention
This invention relates to a process and a device for removing the oxide skin from the cut edges of a thin-walled, metal strip that is cut for preparing strip material for the manufacture of welded pipes.
2. The Prior Art
In the processing of thin-walled metal strips for the manufacture of welded pipes, cable sheaths and outer conductors of coaxial cables, it is a common practice to trim the edges of the strip immediately prior to the welding step. Trimming the strip allows it to be cut to the exact dimensions required for the desired pipe or tube diameter. The oxide on the cut edge is eliminated by the cutting process at the same time, so that the welding process is optimized.
The roller shears used for the trimming require a minimum width to execute the cut. Otherwise, they cannot cut properly. The width of the waste strip normally amounts to 2 to 4 mm on each edge of the metal strip. Therefore, the smaller the desired diameter of the pipe, the greater the ratio of waste to pipe material produced during trimming.
Such losses have been accepted as unavoidable in the past because manufacturers could not rely on being supplied with exactly cut strips and had to provide for exact dimension themselves. This situation has changed recently and operators of pipe welding installations can now cut the strips in their own plants so as to exactly control the quality of the cutting edge and the exact width of the strip. Furthermore, some companies advertising the most exact cutting width have gained their own market share, as they cut large amounts of strip for the manufacture of pipes, so that they were able to acquire great technical expertise. Tests with exactly pre-cut strips have shown that it is possible to weld pipes without any prior trimming. Substantial savings are achieved this way in mass production, and represent a clear competitive edge.
However, it was found in tests that the extremely exact cutting of the starting material fails to satisfy the second important precondition for a good manufacturing result if the product must be stored prior to use. Depending on the material involved, the duration of storage and the storage conditions such as temperature and humidity, an oxide skin of varying thickness develops on the surface of the cuts. This skin interferes with the welding process and may even impede it.